Motor overload relays attempt to model motor heating based on information related to motor operating parameters. Commonly, electronic overload relays monitor motor currents to model heating in the motor stator and rotor based on a thermal model that relates the motor currents to heat generation.
One type of electronic overload relay is a self-powered device. In this type of device, current sensors that provide current measurement information also provide the power to operate the device. By being self-powered, if the motor currents are removed, the overload relay device loses the operating power provided by the current measurement sensors. Motor currents are removed, for example, if the motor is stopped by a control system or if the overload relay “trips” to protect the motor from an undesirable operating condition. Self-powered devices operate on stored energy for the duration of the motor stop, and even during normal operation these devices have a generally low power supply budget.
The relationship between the physical size of materials required in the current sensors, which contributes to the cost of the current sensors, and the operating power available from the sensors, drives a lower operating power budget in self-powered devices. Because the same sensors are used to measure current and derive operating power, increasing power draw from the sensors causes decreased measurement accuracy. In turn, this provides further impetus to maintain a low operating power level.
During the motor stop, when the device is in an unpowered state, it is still necessary to maintain the motor thermal model such that when the motor is restarted the overload protection does not assume that the motor is starting cold (or from an initial unheated state). This function of the overload relay thermal model is called the thermal memory of the device. Some standards, such as National Electrical Manufacturers Association (NEMA) ICS-2, provide requirements for the performance of the thermal memory and create a distinction between volatile and nonvolatile thermal memory based on the characteristics of the function.
Nonvolatile thermal memory means that, following a motor stop of some duration, the motor protection overload relay maintains some thermal memory from the motor thermal model prior to the stop condition. In general, nonvolatile thermal memory is desirable compared to volatile thermal memory because nonvolatile thermal memory maintains the thermal model of the motor during a longer duration stop.
Beyond maintaining the thermal model during the unpowered state, it is also beneficial for the thermal memory function to decrease, when the motor is restarted, the thermal model value based on the duration of the motor stop and to take into account the cooling of the motor by heat dissipation. Existing implementations fail to properly account for motor heat dissipation. For example, existing implementations do not adequately address the low operating power budget available to self-powered devices. In another example, existing implementations also require additional circuitry that otherwise would not be required for operating the device and that only functions to implement the thermal memory method.
What is needed is a low cost implementation of the function of a nonvolatile thermal memory.